1. Field of the Invention
The present invention relates to a beam control device for controlling laser beam of a semiconductor laser included in an optical memory system and in which data can be recorded, played back and/or erased by directing the laser beam onto a magneto-optical disc.
2. Description of the Prior Art
As is well known to those skilled in the art, a magneto-optical disc comprises a structure wherein an amorphous film of rare metal-ferroalloy is deposited on a substrate, such as a glass substrate, by sputtering and which is then covered with a magnetic film having an axis of easy magnetization perpendicular to the surface of the magnetic film.
An optical memory system has a recording head for recording data onto the disc, playing back or erasing the recorded data previously recorded. A semiconductor laser is provided for directing a laser beam onto the disc.
In such an optical memory system, the recording of data is effected according to the following method: a laser beam, focused to a spot of a diameter of about 1 .mu.m, is directed to the magnetic film of the disc to raise the temperature of the portion to which the laser beam is applied in order to reduce the coercive force thereat. At the same time, the direction of magnetization is inverted by applying an auxilliary magnetic field externally of the portion whose temperature has been raised. The erasing method is substantially the same as the recording method just mentioned.
The play-back method for data recorded is as follows: linearly polarized light of the laser beam having an intensity of light weaker than that of the recording beam is applied to the magnetic film of the disc at the location where the data has been recorded. The light reflected therefrom has a certain inclination of polarization due to the magneto-optical effect of the magnetic film (Kerr effect). The inclination of polarization is next converted to an intensity modulated beam of light by an analyzer from which signals are generated and fed as an output from a photo-detector.
As is apparent from the mentioned above, the semiconductor laser is driven at both a high and low levels for recording and playing, respectively.
The semiconductor laser has a temperature dependence in that the intensity of the laser beam varies according to the ambient temperature due to the variation of the threshold current of the semiconductor. If the intensity of laser beam varies during the high level recording mode erroneous information will written into the disc. This lowers the integrity of the optical memory system. Similarly this also applies to the play-back mode. Namely, that when the intensity of laser beam is varied during play-back, the signal to noise (S/N) ratio of play-back signals is lowered to provide wrong output information.
In order to overcome the problems mentioned above, a laser beam controller has been proposed for an optical memory system as shown in FIG. 5.
According to the prior art laser beam controller of FIG. 5, there are provided first and second current sources (b) and (c) for supplying two respectively different driving currents to a semiconductor laser (a). The first current source (b) is utilized for supplying a low power driving current I.sub.1 during play-back of recorded data while during the recording of data, the second current source (c) supplies a high power driving current I.sub.2 to the semiconductor laser in addition to the first current source (b) in order to obtain a laser beam of a high intensity.
When the laser (a) is driven only by the first current source (b), the intensity of laser beam emitted therefrom is detected by a photodetector (d). The output signal thereof is fed as an input, via a pre-amplifier (e), to a sample and hold circuit (f). The sample and hold circuit (f) is controlled by a signal S.sub.1 in such a manner that, when the signal S.sub.1 has a high level, entered data is held therein but is fed therethrough when it receives a low level of S.sub.1. The data signal outputted from the sample and hold circuit (f) is with a reference voltage provided by a standard voltage source (g) in a differential amplifier (h).
The output of differential amplifier (h) is fed as on input to a low-pass filter (i). Low frequency components of the input signal are passed therethrough and are coupled to a power amplifier (j). The power amplifier (j) in turn controls the low current I.sub.1 of the low power current source (b).
Accordingly if the sample and hold signal S.sub.1 is at a low level and the beam intensity of the semiconductor laser (a) is kept constant irrespective to the temperature dependence thereof, this control system is referred to APC (Auto-Power Control).
During a high power driving mode (recording mode), the sample and hold signal S.sub.1 is switched to a high level and the sample and hold circuit (f) holds the data signal and, therefore, the APC is frozen or locked.
Further, when the high power driving or recording mode is chosen, a data-record signal S.sub.D of a high level is applied to an AND gate (k) together with the high level sample and hold signal S.sub.1. The output of the AND gate (k) controls a switching circuit (1). The switching circuit (1) is turned on when the output of AND gate (k) goes to a high level and, at that time, the current I.sub.2 supplied by the high power current source (c) is added to the current I.sub.1 in order to drive the semiconductor laser (a) at the high power level. The reason for making APC freeze is to avoid a possible drop of the beam intensity during the recording and/or erasing mode.
However, APC operates to some extent even in the high power driving mode. This is based on the premise that only the threshold value varies with the ambient temperature, when considering the driving current as a function of the beam intensity curvature characteristic of the semiconductor laser, and that the gradient of said curvature above the threshold value does not vary with the ambient temperature. Namely, if the low power driving current I.sub.1 is controlled so as to have a higher value than the threshold value, APC can be realized even in the high power driving mode by superimposing a constant current and the low power driving current. But these premises are not correct since the gradient of the curve above the threshold does vary with the ambient temperature and use-time.
The sample and hold circuit (f) employed in the APC circuit as shown in FIG. 6 is comprised of a low-pass filter (m) into which the output from a differential pre-amplifier (h) is coupled, a memory means (n) which can store the output V.sub.0 of the low-pass filter (m) and a switching means (o) for switching either the low pass filter (m) or the memory circuit (n) output to a low power driving current source (b). When the play-back mode is desired, the switching means (o) is switched so as to directly connect the low-pass filter (m) to the low power driving current source (b). The differential amplifier (h) outputs a signal V.sub.1 proportional to the difference between the output signal Va from the photo-detector (d) and a predetermined reference voltage Vb. Therefore, APC is obtained as mentioned above.
When the recording mode (high power driving mode) is selected, the switching circuit (o) is switched by the data recording signal S.sub.D so as to connect the output of the memory circuit (n) to the low power driving current source (b). Accordingly, the sample hold circuit (f) outputs a voltage signal V.sub.m which was stored in the memory circuit (n) due to the sample and hold signal S.sub.1. As can clearly be seen, the sample and hold circuit (f) has a first mode in which the output V.sub.0 of the low pass filter (m) is fed out and a second mode during which the output V.sub.m stored in the memory circuit (n) is fed out.
However, this known type of sample and hold circuit (f) has an essential disadvantage in that it is difficult to obtain a quick response when switching from the recording mode to the play-back mode or vice versa since the low pass filter (m) has a relatively slow transition response time.
The response of the low pass filter (m) is shown in FIG. 7. When the operational mode is switched from the recording mode to the play-back mode, the rise of the output voltage Vo of the low pass filter (m) is delayed. Due to this delay in the transition response time, the low power driving current I.sub.1 from the first current source (b) is also delayed and therefore, it takes time until the low power driving current I.sub.1 becomes stabilized.